the assertion in question is in dingliste.cc, line 564. Commenting this out makes the plane disappear when the powerline is build over the shadow (if no ways etc are on the tile). It appears again if it continues flying.
assert(dt->get_typ()==ding_t::aircraft); sollte das beheben.
fixed in rev 3178.
also the costs for removing trees with powerlines were not booked correctly.